<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 300px;
            height: 150px;
            border: 1px solid #000;
            position: relative;
            left: 200px;
            top: 100px;
        }
        .box::before{
            content: "";
            display: inline-block;
            height: 120px;
            width: 120px;
            border: 1px solid #000;
            border-radius: 50%;
            position: absolute;
            left: 50%;
            /* 移动自己的一半 -50% translate(x,y)位移 */
            /* 旋转 */
            transform: translate(-50%,-50%) rotate(45deg);
            background-color: white;
            border-left: 1px solid transparent;
            border-top: 1px solid transparent;
        }
        .box::after{
            content: "";
            display: inline-block;
            height: 80px;
            width: 80px;
            border: 1px solid #000;
            border-radius: 50%;
            position: absolute;
            left: 110px;
            top: -40px;
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>